You can't reforge resilience, which is the main reason why PvP gear is sub-par for PvE.
Most gear comes with two secondary stats, the so-called 'ratings'. Say you're a Death Knight, you'll find that most of your plate armor has the primary stats of strength and stamina. Secondary stats could then be expertise and hit rating, or crit rating, or dodge rating, etc.
At early levels, you might not have any secondary stats, but later on you'll find gear with a bit of hit rating, or crit rating on it. Eventually this becomes more widespread on gear and once you reach Northrend, you'll find that most items have two secondary stats on them.
On PvP gear one of these secondary stats is always resilience (except for some trinkets). Resilience, however, cannot be reforged, so you receive worthless resilience (from a PvE perspective) in exchange for a secondary stat that might have been more beneficial, like dodge rating if you're a tank, or hit rating if you're DPS.
Sometimes, though, it's still better to get gear with a higher item level (iLevel) as it contains more of the primary stats, like stamina and strength, which are more important than the secondary stats.
Did any of that make sense?